1. Field of the Invention
The present invention relates to a process cartridge and an image forming apparatus using the same and more specifically relates to a shaft support mechanism for a rotary member.
2. Description of the Related Art
An image forming apparatus employing an electrophotography system forms an electrostatic latent image on a photosensitive element used as a latent image carrier according to image information, performs a visualization process of the electrostatic latent image using toner supplied from a developing device, and then transfers the resultant image.
There are image forming apparatuses configured to form an image not only in a single color but also in a plurality of colors. These image forming apparatuses are classified into two types with respect to transfer process, i.e., one which subsequently transfers different color toner images onto an intermediate transfer member, and the other which superimposes and transfers different color toner images onto a transfer paper fed from a paper feed device.
For example, a structure called a tandem system in which a belt is used as an intermediate transfer member or as a conveying member that conveys a transfer paper, and a plurality of image forming units are arranged in parallel along the stretched surfaces of the belt are known as the structure that forms an image in the colors.
Each of the image forming units includes a photosensitive element and a charger, a writing device, a developing device, and a cleaning device that perform an image forming process on the photosensitive element. The cleaning device removes the toner being left untransferred on the photosensitive element that has completed the transfer.
A device that works with the photosensitive element included in the image forming unit is a developing device that includes a developing roller as a developer carrier rotating with facing the photosensitive element.
The developing roller has a developer layer with a predetermined thickness on its surface and performs a visualization process of electrostatic latent images by making the developer layer contact with the electrostatic latent images supported on the surface of the photosensitive element. In such a manner, the position of the developing roller is determined with a narrow gap formed between the developing roller and the surface of the photosensitive element. Moreover, the developing roller and the photosensitive element are geared together by intermeshing gears provided at their rotation shafts to rotate the developing roller and the photosensitive element together.
To optimize the determination of the position of the developing roller relative to the photosensitive element, Japanese Patent Application Laid-open No. 2008-139818 discloses, for example, a structure that utilizes the pressing force derived from the own weight of the developing roller by positioning the developing roller above the photosensitive element and obtained from a component of force, which is directed to the surface of the photosensitive element, of rotating force arising from the rotation of the developing roller.
Japanese Patent Application Laid-open No. 2008-139818 discloses structures of the shaft supporting units of rotation shafts as illustrated in FIGS. 4 and 5 that are also used for explaining an embodiment of the present invention.
FIG. 4 illustrates a structure that includes a rotatable bearing 18a working with a shaft end 6h3 of a support shaft 6h0 fitting with a shaft center portion of a roller like developer carrier 6h and a block body 18b coming in contact with the peripheral surface of the bearing 18a. The structure also includes a pressing spring 19 that is arranged between the block body 18b and a guiding unit 18c for guiding the bearing 18a in a direction in which the developer carrier 6h comes in contact with and is separated from a photosensitive element as a latent image carrier 6a and that has a tendency to push the developer carrier 6h against the photosensitive element.
FIG. 5 illustrates a structure that includes a locking portion on which one end of the spring 19 is loaded at a portion of the peripheral surface of the locking portion and that does not rotate as a structure of a bearing (represented as a reference numeral 18a′ for convenience). With the structure, the shaft end 6h3 of the support shaft 6h0 is inserted in the bearing 18a′ to be rotatable.
As illustrated in FIG. 12, force relationship regarding pressing of the developing roller against the surface of the photosensitive element at the shaft supporting unit of the developing roller included in the bearing structure described above is explained using acting force at an engagement position of the gears. In FIG. 12, vectors represented as arrows are force that occurs at the engagement position. However, for convenience, the vectors are illustrated with their base points being at the shaft center of the bearing 18a in order to facilitate understanding of the force relationship acting between the bearing 18a and the guiding unit 18c. The lengths of the arrows indicating the strength of force of the vectors are appropriately changed and are not in a proportional relationship with the actual values.
In FIG. 12, a gear (gear represented as a reference numeral 6h-G in FIG. 3 illustrating an embodiment of the present invention) provided at a driving side end of the developer carrier 6h employing a developing roller is engaged with a gear (gear represented as a reference numeral 6a-G in FIG. 3 illustrating the embodiment) provided at a driving side end of the photosensitive drum 6a. Therefore, when a gear (not illustrated) positioning at the image forming apparatus side rotates the photosensitive drum a in an arrow a direction, the developer carrier 6h rotates in an arrow b direction.
A driving force 6F1 of the developer carrier 6h is a vector in a direction 20 degrees from a tangential direction at a position where the photosensitive drum 6a makes contact with the developer carrier 6h because the pressure angle of the gear is 20 degrees.
The developer carrier 6h presses the bearing (represented as a reference numeral 18a for convenience) into which the rotation support shaft of the developer carrier 6h is inserted against the guide (represented as a reference numeral 18c indicated in FIG. 4 for convenience) by the driving force 6F1. The angle formed by the wall surface of the guiding unit 18c and the direction of the driving force 6F1 is designed to be an obtuse angle (90°<) in a direction of the photosensitive drum 6a. 
With this, the driving force 6F1 at a contact position of the bearing 18a and the guiding unit 18c is divided into a component force 6F1a for driving a developer carrier and a component force 6F1b for driving a developer carrier. The component force 6F1a for driving a developer carrier acts in a direction perpendicular to the wall surface of the guiding unit 18c (this component of force corresponds to the component of force of the rotating force in a tangential direction given to the developer carrier 6h), and the component force 6F1b for driving a developer carrier acts in a direction parallel to the wall surface of the guiding unit 18c. 
A force of an own weight 6F2 of the developing device itself is divided at two portions where the developing device makes contact with the guiding unit 18c and the photosensitive drum 6a. For brevity, only a component force 6F2b of own weight of the developing device in a direction at an angle of the wall of the guiding unit 18c is illustrated.
A resultant force of the component force 6F1b for driving a developer carrier, the component force 6F2b of own weight of the developing device, and a pressing force 6F3 by the pressing spring (represented as a reference numeral 19 for convenience) provided in order to push the developer carrier 6h against the photosensitive drum 6a is divided into components of force. One of the components of force at a contact position of the developer carrier 6h and the photosensitive drum 6a in a direction connecting the shafts of the developer carrier 6h and the photosensitive drum 6a becomes a force F by which the developer carrier 6h is pushed against the photosensitive drum 6a. 
As is evident from the explanation given above, the bearing 18a maintains a relationship of pushing the developer carrier 6h against the photosensitive drum 6a by contacting the bearing 18a with the wall surface of the guiding unit 18c at a portion of the peripheral surface of the bearing 18a. The bearing 18a is required for a function to maintain an opposed state of the developer carrier 6h against the photosensitive drum 6a in a proper opposed relationship while absorbing periodical shaft deflection due to eccentricity generated in the photosensitive drum 6a and the developer carrier 6h themselves or due to displacement of the shaft centers of the gears. For this reason, the bearing 18a is periodically reciprocated (a movement in a direction in which the developer carrier comes in contact with and is separated from the photosensitive drum) at the guiding unit 18c to absorb the shaft deflection. Such a reciprocating movement is a movement of coming in contact with or being separated from the wall surfaces of the guiding unit. Accordingly, while rotating, the bearing 18a periodically slides on the wall surface of the guiding unit 18c when they come in contact, and while not rotating, the bearing 18a repeats sliding due to the reciprocation.
The repetition of the periodical sliding of the bearing may deform or break a part of the wall surface of the guiding unit 18c. 
When a part of the wall surface is deformed or broken, the distance between the shafts of the photosensitive drum and the developer carrier are changed. As a result, the supplying state of the developer supported by the developer carrier is changed from its proper state. In other words, the contact state of the development layer with the electrostatic latent images may vary. This may change the supplying state of toner to the electrostatic latent images, causing the excess of density and the insufficiency of density.